1. Field of the Invention
The preferred embodiments are directed to using a nano-localized light source to measure physical properties of a sample, and more particularly, to a method and apparatus of making nano-imaging and spectroscopy measurements using an atomic force microscope operating in either contact or a low amplitude mode, with the tip apex of the probe functioning as the nano-localized source.
2. Description of Related Art
The interaction between a sample under test and radiated energy can be monitored to yield information concerning the sample. In spectroscopy, dispersion of light from a sample into its component energies can be measured and, for example, intensity plotted as a function of wavelength. By performing this dissection and analysis of the dispersed light, users can determine the physical properties of the sample, such as temperature, mass and composition.
Notably, making spectroscopic measurements with a spatial resolution on the nanoscale is continuing to improve. But, as noted by Berweger et al, in Adiabatic Tip-plasmon Focusing for Nano-Raman Spectroscopy, J. Phys. Chem. Letters (November 2010) (“Berweger I”), the entirety of which is hereby incorporated by reference, despite ongoing progress in the development of imaging techniques with spatial resolution beyond the diffraction limit, simultaneous spectroscopic implementations delivering chemical specificity and sensitivity on the molecular level have remained challenging. Far-field localization techniques can achieve spatial resolution down to about 20 nm by point-spread function reconstruction, but typically rely on fluorescence from discrete molecular or quantum dot emitters, with limited chemically specific information. Scanning near-field optical microscopy (SNOM) provides sub-diffraction-limited resolution through the use of tapered fibers or hollow waveguide tips. However, aperture-limited and wavelength-dependent fiber throughput reduces sensitivity, generally making SNOM unsuitable for spectroscopic techniques that have low intrinsic signal levels.
In scattering-type SNOM (s-SNOM) external illumination of a sharp (metallic or semi-conducting) probe tip can enhance sensitivity, spectral range, and spatial resolution, as noted in Berweger. Chemical specificity can be obtained through the implementation of, for example, IR vibrational s-SNOM, tip-enhanced coherent anti-Stokes Raman spectroscopy (CARS), or tip-enhanced Raman scattering (TERS). Here the antenna or plasmon resonances of the (noble) metal tips can provide the necessary field enhancement for even single-molecule sensitivity.
In the standard implementation, however, the direct illumination of the tip apex results in a three-to-four orders of magnitude loss in excitation efficiency, related to the mode mismatch between the diffraction-limited far-field excitation focus and the desired tens of nanometers near-field localization, as determined by the tip apex radius. The resulting loss of sensitivity, together with a far-field background signal, often limit contrast and may cause imaging artifacts, presenting challenges for the general implementation of a wider range of spectroscopic techniques in s-SNOM.
A general solution for optical nano-imaging and spectroscopy thus requires a true nano-localized light source. While this can be achieved through a nanoscopic emitter in the form of a single molecule, quantum dot, or nano structure at the apex of a tip, that approach relies on the quantum efficiency and spectral characteristics of the emitter, and the difficulties of overcoming the intrinsic background and sensitivity limitations with unmatched far-field mode excitation remain.
In one known technique, a nanoemitter is generated through nonlocal excitation, taking advantage of the effective tip cone radius-dependent index of refraction n(r) experienced by a surface plasmon polariton (SPP) propagating along the shaft of a noble metal tip. The resulting propagation-induced adiabatic SPP focusing into the tip apex region is due to the continuous transformation of the surface mode size. This approach allows for SPP coupling spatially separated from the tip apex, and subsequent probe apex excitation via the propagating SPP, with tens of nanometers field confinement over a broad spectral range with high focusing efficiency.
As further noted in Berweger, the use of a photonic crystal microresonator as a coupling element on a tip has previously been used to demonstrate TERS. As explained by the authors, it was believed that the geometric constraints of the cantilever-based design made the study of opaque samples difficult, and the collinear excitation and residual hole-array transmission did not yet fully eliminate the far-field background. In a known analogous but simplified approach, a grating-coupler was employed to launch SPP modes onto the shaft of monolithic gold (Au) tips. The conical tips with two-stage optical mode matching of the far-field. SPP coupling and the mechanisms of adiabatic SPP field concentration into the tip apex represent a unique optical antenna concept for the efficient far-field transduction into nanoscale excitation.
As shown in FIG. 1, the nanofocusing process can be implemented by employing an etched tip and using side-illumination grating coupling. A schematic image of an electrochemically etched Au probe tip 200 with plasmonic grating 206 is shown. An optical image 208 is superimposed on the illustration showing the illumination of grating 206 with a far-field focus for launching SPPs, which then propagate non-radiatively along the shaft or body 202 of tip 200, with corresponding localized emission from the nanofocused field at the tip apex 204 (the energy is collected and concentrated at the apex as a localized source, represented schematically with a dimension “d”). The apex-emitted light follows a cos2 (θ) polarization dependence, as expected for a nanoscopic dipolar emitter located at the tip apex. These results confirm the expected mode filtering of the nanofocusing process, as only the radially symmetric m=0 SPP mode will produce purely axial dipole emission, due to destructive interference of the radial polarization components.
One promising technology for improving spectroscopic measurement performance is scanning probe microscopy. Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and causing the tip to interact with the surface of a sample with appropriate forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
A typical AFM system is shown schematically in FIG. 2. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15. Scanner 24 generates relative motion between the probe 14 and sample 22 while the probe-sample interaction is measured. In this way images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be an assembly of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY scanner that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other surface property of the sample as described, e.g., in Hansrna et al. U.S. Pat. No. RE 34,489; flings et al. U.S. Pat. No 5,266,801; and Elings est al, U.S. Pat. No. 5,412,980.
In a common configuration, probe 14 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 14 at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other motion of cantilever 15. Probe 14 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 14 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14, but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe.
Often, a selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. In general, controller 20 generates control signals to maintain a relative constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
A workstation is also provided, in the controller 20, and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform point selection, curve fitting, and distance determining operations.
AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving either the sample or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an “x-y” plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the “z” direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term “generally parallel.” In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the “tracking force,” i.e. the force resulting from tip/sample interaction. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored, and used as data to characterize the sample. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., “atomic force microscopy.”
Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid, or vacuum, by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research.
In one embodiment employing an AFM for optical excitation and detection, the system provides side-on illumination of the tip shaft (body) of a tip, supporting a grating. The electrochemically etched tips may be mounted onto an AFM quartz tuning fork, and the grating is fabricated via focused ion beam (FIB) milling. But such known systems have limitations. For example, the tip-sample coupling relationship is complex and can, for example, result in a loss of excitation enhancement due to the broad k-vector distribution at the apex allowing for launching propagating SPPs at the sample surface. This is suggested by reduced fundamental apex emission often observed upon approach of an Au tip surface. Also, use of electrochemically etched tips is limited to shear force microscopy (tuning fork tips—difficult to implement tip-sample modulation with these). As a result, the AFM modes available for simultaneous and sensitive topographic, phase, and other state of the art AFM functions is severely limited. Electrochemically etched tips are also difficult to mass produce, and their reproducibility is limited. In addition, the materials that can be used to produce the tips are limited. Electrochemical etching allows for the fabrication of sharp tips with only certain metals, such as gold, silver, or tungsten. And with large amplitude AFM modes, the tip is removed from the optical near-field during spectral acquisition, thus producing a weaker spectroscopic signal as the tip on average is further away from the sample.
As a result, improvements were needed to expand the range and efficiency of performing optical nanoimaging and spectroscopy on the nanoscale.